U.S. patent number 10,644,214 [Application Number 15/088,765] was granted by the patent office on 2020-05-05 for thermoelectric nanocomposite and process of producing the same.
This patent grant is currently assigned to Hyundai Motor Company, Industry-Academic Cooperation Foundation, Yonsei University. The grantee listed for this patent is Hyundai Motor Company, Industry-Academic Cooperation Foundation, Yonsei University. Invention is credited to Sung Mee Cho, Byung Wook Kim, Gwan Sik Kim, Jin Woo Kwak, Han Saem Lee, Woo Young Lee, In Woong Lyo, Su Jung Noh, Kyong Hwa Song.
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United States Patent |
10,644,214 |
Kim , et al. |
May 5, 2020 |
Thermoelectric nanocomposite and process of producing the same
Abstract
A thermoelectric nanocomposite is provided. The thermoelectric
nanocomposite includes: a matrix having n-type semiconductor
characteristics and comprising Mg, Si, Al, and Bi components, and a
nanoinclusion comprising Bi and Mg components. The thermoelectric
nanocomposite has significantly increased thermoelectric energy
conversion efficiency by simultaneously having an increased Seebeck
coefficient and a decreased thermal conductivity, such that the
thermoelectric nanocomposite is usefully used to implement a
thermoelectric device having high efficiency.
Inventors: |
Kim; Byung Wook (Gyeonggi-do,
KR), Song; Kyong Hwa (Seoul, KR), Lee; Han
Saem (Seoul, KR), Kwak; Jin Woo
(Gyeongsangbuk-do, KR), Lyo; In Woong (Gyeonggi,
KR), Noh; Su Jung (Seoul, KR), Lee; Woo
Young (Seoul, KR), Kim; Gwan Sik (Gyeonggi-do,
KR), Cho; Sung Mee (Seoul, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hyundai Motor Company
Industry-Academic Cooperation Foundation, Yonsei
University |
Seoul
Seoul |
N/A
N/A |
KR
KR |
|
|
Assignee: |
Hyundai Motor Company (Seoul,
KR)
Industry-Academic Cooperation Foundation, Yonsei University
(Seoul, KR)
|
Family
ID: |
58456597 |
Appl.
No.: |
15/088,765 |
Filed: |
April 1, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20170110644 A1 |
Apr 20, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
Oct 14, 2015 [KR] |
|
|
10-2015-0143638 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C
23/00 (20130101); H01L 35/34 (20130101); H01L
35/18 (20130101) |
Current International
Class: |
H01L
35/18 (20060101); H01L 35/34 (20060101); C22C
23/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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104335327 |
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Feb 2015 |
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CN |
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104349854 |
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Feb 2015 |
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CN |
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2000-164940 |
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Jun 2000 |
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JP |
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2008-523614 |
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Jul 2008 |
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JP |
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2008-305919 |
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Dec 2008 |
|
JP |
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2011-134988 |
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Jul 2011 |
|
JP |
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2012-253229 |
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Dec 2012 |
|
JP |
|
5230206 |
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Jul 2013 |
|
JP |
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2007-0108853 |
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Nov 2007 |
|
KR |
|
10-2012-0125697 |
|
Nov 2012 |
|
KR |
|
10-2013-0084120 |
|
Jul 2013 |
|
KR |
|
10-1530376 |
|
Jun 2015 |
|
KR |
|
Other References
S Fiameni, A. Famengo, F. Agresti, S. Boldrini, S. Battiston, M.
Saleemi, et al.Effect of synthesis and sintering conditions on the
thermoelectric properties of n-doped Mg2Si. J. Electron. Mater.
(2014), pp. 2301-2306.). (Year: 2014). cited by examiner .
Niu, et al., "Thermodynamic assessment of the Bi-Mg binary system",
Acta Metallurgica Sinica, vol. 25, No. 1, Feb. 1, 2012, pp. 19-28.
cited by applicant.
|
Primary Examiner: Dunn; Colleen P
Assistant Examiner: Liang; Anthony M
Attorney, Agent or Firm: Mintz Levin Cohn Ferris Glovsky and
Popeo, P.C. Corless; Peter F.
Claims
What is claimed is:
1. A thermoelectric nanocomposite, comprising: a matrix having
n-type semiconductor characteristics; and a nanoinclusion
comprising Bi and Mg, wherein the nanoinclusion is embedded in the
matrix, wherein the matrix comprises
Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.03, and the nanoinclusion
comprises Bi.sub.2Mg.sub.3, a weight ratio of the nanoinclusion to
the matrix being 2.6.+-.0.26%.
2. The thermoelectric nanocomposite according to claim 1, wherein
the nanoinclusion has an average particle size of 1 to 500 nm.
3. The thermoelectric nanocomposite according to claim 1, wherein a
density of a phase boundary between the matrix and nanoinclusion is
of 350 to 4200 cm.sup.2/cm.sup.3.
4. The thermoelectric nanocomposite according to claim 1, wherein
the nanoinclusion has a particle size less than a mean free path of
the matrix.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application is based on and claims the benefit of priority to
Korean Patent Application No. 10-2015-0143638, filed on Oct. 14,
2015 in the Korean Intellectual Property Office, the disclosure of
which is incorporated herein in its entirety by reference.
TECHNICAL FIELD
The present invention relates to a thermoelectric nanocomposite
having high thermoelectric conversion efficiency, and more
particularly, to a thermoelectric nanocomposite containing a
thermoelectric matrix and an inclusion formed in nanometer
size.
BACKGROUND
A thermoelectric phenomenon, which is a reversible and direct
energy conversion phenomenon between heat and electricity through a
solid-state material, is a phenomenon generated by movement of
electrons or holes in a thermoelectric material. The thermoelectric
phenomenon can be explained with a Peltier effect in which heat is
emitted or absorbed when applying current from the outside, a
Seebeck effect in which electromotive force is generated from a
difference in temperature between both ends of a material, and a
Thomson effect in which heat is emitted or absorbed when a current
flows in a material having a predetermined temperature
gradient.
When using the Peltier effect, a cooling system which does not
require a gas compressor and refrigerant gas may be implemented.
Further, when using the Seebeck effect, heat generated in a
computer, a vehicle engine, or the like, or waste heat generated in
various industries may be converted into electric energy.
Recently, as a necessity for a technology of improving energy usage
efficiency including vehicle fuel efficiency has increased, an
interest in a power generation system using the thermoelectric
material has also increased. For example, thermoelectric cooling
and thermoelectric power generation efficiency may be directly
connected with performance of the thermoelectric material, and to
overcome a current limitation of the thermoelectric material
restrictively used in a small and special cooling field,
development of a high performance material is demanded.
In the related art, thermoelectric energy conversion efficiency
indicating performance of the thermoelectric material is presented
by dimensionless figure of merit (ZT) represented by the following
Equation 1.
.times..sigma..times..times..kappa..times..times. ##EQU00001##
In Equation 1, S is a Seebeck coefficient, .sigma. is an electrical
conductivity, T is an absolute temperature, and .kappa. is a
thermal conductivity.
To increase thermoelectric efficiency (i.e. ZT), a thermoelectric
material simultaneously having a high Seebeck coefficient, a high
electrical conductivity, and a low thermal conductivity may be
required. However, since the Seebeck coefficient and the electrical
conductivity have a trade-off relationship with a carrier
concentration and the electrical conductivity and the thermal
conductivity are not independent variables but are affected by each
other, it may be complicated to implement a material having high
ZT.
One of important strategies for improving performance of the
thermoelectric material which is to manufacture a nanocomposite,
may be manufacturing a nanograin structure of which a density of a
grain boundary is increased by decreasing a grain size to a nano
size or manufacturing a nanocomposite in which a phase boundary
between a thermoelectric matrix and a secondary phase is formed by
introducing the secondary phase having a nano size. In particular,
thermal conductivity may be decreased by increasing phonon
scattering in the grain boundary and the phase boundary, and the
trade-off relationship between the Seebeck coefficient and the
electrical conductivity is broken by a carrier filtering effect,
thereby making it possible to improve ZT.
A nanostructure may be manufactured in a form of a zero-dimensional
quantum dot, a one-dimensional nanowire, a two-dimensional nano
plate, and a superlattice thin film, but a nanostructure material
providing high ZT in a bulk form is required for actual
application.
The above information disclosed in this Background section is
merely for enhancement of understanding of the background of the
invention and therefore it may contain information that does not
form the prior art that is already known in this country to a
person of ordinary skill in the art.
SUMMARY
In preferred aspects, the present invention provides a
thermoelectric material comprising an Mg--Si component.
Accordingly, the thermoelectric material of the invention may
provide high applicability as a thermoelectric power generation
material at a medium temperature. For instance, the thermoelectric
material may be used for a vehicle, due to characteristics such as
non-toxicity, a cheap base material, a low density, and the like.
In addition, the Mg--Si based thermoelectric material may improve
thermoelectric performance by simultaneously implementing a low
thermal conductivity and a high power factor, for example, as the
thermoelectric performance value may be obtained by multiplying the
electrical conductivity and the square of the Seebeck coefficient
together.
In one aspect, the present invention provides a thermoelectric
nanocomposite that may include a matrix and an inclusion. In
particular, the matrix may have n-type semiconductor
characteristics and may include Mg, Si, Al, and Bi components and
the inclusion may be formed in nanoscale and comprise Bi and Mg
components.
The term "n-type semiconductor" or "n-type", as used herein, refers
to a material or substance that is created by adding pentavalent
impurities or dopants (e.g. phosphorus (P), arsenic (As), and
antimony (Sb)), which can donate a free electron A to a
semiconductor (substance matrix). As such, n-type semiconductor may
include more charge carriers, or electrons available in the
material for electric conductivity and thermoelectric effect.
According to an exemplary embodiment of the present invention, a
thermoelectric nanocomposite may comprise: a matrix comprising
magnesium (Mg), silicon (Si), aluminum (Al), and bismuth (Bi)
components; and an inclusion comprising bismuth (Bi) and magnesium
(Mg) components. In particular, the matrix may include the
components represented by the following Chemical Formula 1 and the
inclusion may include the components represented by the following
Chemical Formula 2. Mg.sub.2-xAl.sub.xSi.sub.1-yBi.sub.y, Chemical
Formula 1
where 0.ltoreq.x.ltoreq.0.04, and 0.ltoreq.y.ltoreq.0.04.
Bi.sub.2Mg.sub.3.+-.z, Chemical Formula 2
where 0.ltoreq.z.ltoreq.0.1.
The matrix may further include Sn. For instance, the matrix may be
represented by the following Chemical Formula 3.
Mg.sub.2-xAl.sub.xSi.sub.1-y-wBi.sub.ySn.sub.w, Chemical Formula
3
Where 0.ltoreq.x.ltoreq.0.04, 0.ltoreq.y.ltoreq.0.04, and
0.ltoreq.w.ltoreq.0.5.
The inclusion may have an average particle size of about 1 to 500
nm. Unless otherwise indicated, the inclusion in the above range
can be referred to as a nanoinclusion. The inclusion or the
nanoinclusion may be included at a content of about 0.1 to 4.0
parts by weight based on 100 parts by weight of the matrix.
The term "inclusion" as used herein refers to particles or
distinctive substances (e.g. metal particle or metallic compound)
formed as being embedded in other materials (e.g. matrix).
Preferably, the inclusion may be formed to have distinctive
boundaries between the inclusion body and the matrix, to provide
additional properties to the matrix. For instance, the components
of the thermoelectric nanocomposite as described herein may form
inclusions, such as metallic compound of Chemical Formula 2,
comprising magnesium (Mg), silicon (Si), aluminum (Al), and bismuth
(Bi) components. Accordingly, those inclusions may be formed in
distinctive particles having ranges of sizes. In particular, the
inclusions may be formed in nanoscale size, which suitably provide
physical or chemical properties (e.g. the increased Seebeck
coefficient) to the thus formed nanocomposite material.
For example, the matrix may include
Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.03, and the nanoinclusion
may include Bi.sub.2Mg.sub.3. A weight ratio of the nanoinclusion
to the matrix may be about 2.6%.
Alternatively, the matrix may include
Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.04, and the nanoinclusion
may include Bi.sub.2Mg.sub.3. The weight ratio of the nanoinclusion
to the matrix may be about 4.0%.
In other example, the matrix may include
Mg.sub.1.96Al.sub.0.04Si.sub.0.97Bi.sub.0.03, and the nanoinclusion
may include Bi.sub.2Mg.sub.3. The weight ratio of the nanoinclusion
to the matrix may be about 1.7%.
In further example, the matrix may include
Mg.sub.1.96Al.sub.0.04Si.sub.0.96Bi.sub.0.04, and the nanoinclusion
may include Bi.sub.2Mg.sub.3. The weight ratio of the nanoinclusion
to the matrix may be about 2.4%.
A density of a phase boundary between the matrix phase and the
secondary phase may be about 350 to 4200 cm.sup.2/cm.sup.3.
Preferably, the nanoinclusion may have a particle size less than a
mean free path of the matrix.
As used herein, the "mean free path" or "carrier mean free path"
refers to an average distance traveled by a moving particle (e.g.
an atom, a molecule, a photon, or an electron) along its trajectory
before impacts (collisions), which modify its direction or energy
or other particle properties. For example, the mean free path may
be the average distance of particles in the matrix, and
particularly, the mean free path may be less than a sized of the
inclusion. In another aspect, the present invention provides a
process of producing the thermoelectric nanocomposite as described
herein. According to an exemplary embodiment of the present
invention, a process of producing a thermoelectric nanocomposite
may include: (a) preparing a nanocomposite base material powder;
and (b) sintering the nanocomposite base material powder obtained
in step (a) to obtain a thermoelectric nanocomposite.
The nanocomposite base material powder may be prepared by steps
comprising: mixing precursors; and compacting the mixed precursors
at a pressure of 10 to 90 MPa at a temperature of about 10 to 30 to
form a pellet; and heat treating the pellet under vacuum.
In addition, the heated pellet may be grounded by a ball milling
method, an attrition milling method, a high energy milling method,
a zet milling method, or a grinding method using a mortar.
The nanocomposite base material powder obtained in step (a) may be
sintered by steps comprising: depositing the nanocomposite base
material powder obtained in step (a) into a mold having a
predetermined shape; and molding the composite base material powder
at a temperature of about 500 to 900.degree. C., and a pressure of
about 30 to 300 MPa.
Alternatively, the nanocomposite base material powder obtained in
step (a) may be sintered by steps comprising: applying a current of
about 50 to 500 A at a pressure of about 30 to 300 MPa.
Alternatively, the nanocomposite base material powder obtained in
step (a) may be sintered by steps comprising: applying a
temperature of about 500 to 900.degree. C. to the nanocomposite
base material powder obtained in step (a); and simultaneously
sintering and molding the thermoelectric nanocomposite base
material powder.
Preferably, an inclusion may be formed or precipitated in the step
(b) of sintering.
Further provided in the present invention is a vehicle that may
comprise the thermoelectric nanocomposite as described herein.
Other aspect of the invention is disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will be more apparent from the following detailed
description taken in conjunction with the accompanying
drawings.
FIG. 1 illustrates exemplary structures of
Mg.sub.2-xAl.sub.xSi.sub.1-yBi.sub.y as an exemplary matrix phase
and an exemplary Bi.sub.2Mg.sub.3 nanoinclusion of an exemplary
thermoelectric nanocomposite according to an exemplary embodiment
of the present invention;
FIG. 2 is an exemplary scanning electron microscope (SEM) image
showing a microstructure of an exemplary
Mg.sub.2-xAl.sub.xSi.sub.1-yBi.sub.y matrix and an exemplary
Bi.sub.2Mg.sub.3 nanoinclusion of an exemplary thermoelectric
material prepared in Example 1 according to an exemplary embodiment
of the present invention;
FIG. 3 illustrates results obtained by measuring electrical
conductivities of thermoelectric materials prepared in Comparative
Example and Examples 1 to 4 according to exemplary embodiments of
the present invention;
FIG. 4 illustrates results obtained by measuring Seebeck
coefficients of the thermoelectric materials prepared in
Comparative Example and Examples 1 to 4 according to exemplary
embodiments of the present invention;
FIG. 5 illustrates results obtained by measuring power factors of
the thermoelectric materials prepared in Comparative Example and
Examples 1 to 4 according to exemplary embodiments of the present
invention;
FIG. 6 is a Pisarenko plot illustrating relationships between
carrier concentrations and Seebeck coefficients of the
thermoelectric materials prepared in Comparative Example and
Examples 1 to 4 according to exemplary embodiments of the present
invention;
FIG. 7 illustrates results obtained by measuring thermal
conductivities of the thermoelectric materials prepared in
Comparative Example and Examples 1 to 4 according to exemplary
embodiments of the present invention;
FIG. 8 illustrates results obtained by measuring lattice thermal
conductivities of the thermoelectric materials prepared in
Comparative Example and Examples 1 to 4 according to exemplary
embodiments of the present invention; and
FIG. 9 illustrates results obtained by measuring figures of merit
(ZTs) of the thermoelectric materials prepared in Comparative
Example and Examples 1 to 4 according to exemplary embodiments of
the present invention.
DETAILED DESCRIPTION
The terminology used herein is for the purpose of describing
particular exemplary embodiments only and is not intended to be
limiting of the invention. As used herein, the singular forms "a",
"an" and "the" are intended to include the plural forms as well,
unless the context clearly indicates otherwise. It will be further
understood that the terms "comprises" and/or "comprising," when
used in this specification, specify the presence of stated
features, integers, steps, operations, elements, and/or components,
but do not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or
groups thereof. As used herein, the term "and/or" includes any and
all combinations of one or more of the associated listed items.
Unless specifically stated or obvious from context, as used herein,
the term "about" is understood as within a range of normal
tolerance in the art, for example within 2 standard deviations of
the mean. "About" can be understood as within 10%, 9%, 8%, 7%, 6%,
5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated
value. Unless otherwise clear from the context, all numerical
values provided herein are modified by the term "about."
It is understood that the term "vehicle" or "vehicular" or other
similar term as used herein is inclusive of motor vehicles in
general such as passenger automobiles including sports utility
vehicles (SUV), buses, trucks, various commercial vehicles,
watercraft including a variety of boats and ships, aircraft, and
the like, and includes hybrid vehicles, electric vehicles, plug-in
hybrid electric vehicles, hydrogen-powered vehicles and other
alternative fuel vehicles (e.g. fuels derived from resources other
than petroleum). As referred to herein, a hybrid vehicle is a
vehicle that has two or more sources of power, for example both
gasoline-powered and electric-powered vehicles.
According to an exemplary embodiment of the present invention, a
thermoelectric nanocomposite may include: a matrix comprising Mg,
Si, Al, and Bi components; and an inclusion (nanoinclusion)
comprising Bi and Mg components.
The matrix may be represented by the following Chemical Formula 1:
Mg.sub.2-xAl.sub.xSi.sub.1-yBi.sub.y, where 0.ltoreq.x.ltoreq.0.04,
and 0.ltoreq.y.ltoreq.0.04. Chemical Formula 1
Preferably, the matrix of the above Chemical Formula may have
n-type semiconductor characteristics.
The inclusion may be represented by the following Chemical Formula
2: Bi.sub.2Mg.sub.3.+-.z, where 0.ltoreq.z.ltoreq.0.1. Chemical
Formula 2
In the thermoelectric nanocomposite, the inclusion as a secondary
phase may be a nanoinclusion having a size in nanometers, for
example, from about 1 nm to about 500 nm. The nanoinclusion may be
dispersed in the matrix having n-type semiconductor characteristics
to be embedded therein, and thus, a new interface between the
matrix and the nanoinclusion may be formed, thereby having an
effect of actually introducing a nanostructure. As a consequence,
phonon scattering may be increased at the interface, thereby
decreasing a lattice thermal conductivity. Further, since
compositions of the matrix and the nanoinclusion in the
thermoelectric nanocomposite may be different from each other,
carriers may be selectively transported by adjusting the
compositions of the matrix phase and the secondary phase. In other
words, an energy barrier height in the interface between the matrix
phase and the secondary phase may be adjusted by adjusting the
compositions of the matrix phase and the secondary phase. A carrier
filtering effect of selectively transporting only carriers
significantly contributing to power factor (e.g. S.sup.2.sigma.)
may be obtained by adjusting the energy barrier height. The Seebeck
coefficient (S) may be increased by the carrier filtering effect,
and as a result, properties (e.g., thermoelectric energy conversion
efficiency, carrier filtering effect and the like) may be
improved.
The inclusion or nanoinclusion as the secondary phase may be
present in an intragrain region of the matrix in the thermoelectric
nanocomposite. Further, the secondary phase may be present in a
grain boundary of the matrix. The secondary phase or inclusion may
be formed as Mg is volatilized in a process of heat treating base
material powder.
Preferably, the secondary phase may include Bi.sub.2Mg.sub.3
compound.
The thermoelectric nanocomposite, the inclusion having a nano size
refers to a secondary phase having an average particle size less
than about 1 .mu.m. For example, an average particle size of the
inclusion may be of about 1 to 900 nm, of about 1 to 700 nm, or
particularly of about 1 to 500 nm. The size of the inclusion may be
less than a carrier mean free path of the matrix.
As used herein, the "mean free path" or "carrier mean free path"
refers to an average distance traveled by a moving particle (e.g.
an atom, a molecule, a photon, or an electron) along its trajectory
before impacts (collisions), which modify its direction or energy
or other particle properties. For example, the mean free path may
be the average distance of particles in the matrix, and
particularly, the mean free path may be less than a sized of the
inclusion. When the average particle size of the secondary phase
(inclusion) is greater than the carrier mean free path of the
matrix phase, the carriers as well as phonons may scatter, and
thus, electrical conductivity may be decreased, which may limit a
ZT increasing effect.
The nanoinclusion as of the secondary phase may be included in a
content range of about 0.1 to 4.0 parts by weight based on 100
parts by weight of the matrix. The thermoelectric energy conversion
efficiency (figure of merit, ZT) of the thermoelectric
nanocomposite may be further improved in the above-mentioned
content range.
The thermoelectric nanocomposite may provide a significantly
increased ZT value compared to a conventional thermoelectric
material with a structure of the secondary phase or nanoinclusion
dispersed in the matrix as described above. Preferably, the
thermoelectric nanocomposite may have a figure of merit (ZT) of
about 1.0 or greater at a temperature of about 873K.
In the thermoelectric nanocomposite according to the present
invention, the secondary phase may be formed as the nanoinclusion
in the matrix, thereby forming a high-density phase boundary
between the matrix phase and the secondary phase. A density of the
phase boundary may be of about 350 to 4200 cm.sup.2/cm.sup.3.
The thermoelectric nanocomposite according to the present invention
may have a Seebeck coefficient increased due to the carrier
filtering effect by band-bending in the phase boundary, and have a
lattice thermal conductivity decreased by the phonon scattering in
this phase boundary, such that the thermoelectric nanocomposite may
have improved ZT characteristics by simultaneously implementing an
increase in Seebeck coefficient and a decrease in thermal
conductivity.
The thermoelectric nanocomposite may be formed in a bulk phase.
Further, the bulk-phase thermoelectric nanocomposite may be
manufactured into a pressure-sintered material prepared by
compressing and sintering composite base material powder.
Further provided is a process of producing a thermoelectric
nanocomposite including: (a) preparing nanocomposite base material
powder; and (b) sintering the nanocomposite base material powder
obtained in step (a) to obtain a thermoelectric nanocomposite.
Step (a) may include preparing the nanocomposite base material
powder for producing the thermoelectric nanocomposite.
For example, precursors (e.g., metal raw materials) of a
thermoelectric material may be mixed with each other at a
predetermined ratio, and then cold-compacted at a temperature of
about 10 to 30 at a pressure of about 10 to 90 MPa, thereby
preparing a precursor pellet. Subsequently, the precursor pellet
may be heat treated under vacuum for 2 to 10 hours, to prepare the
thermoelectric nanocomposite. However, the conditions are not
necessarily limited thereto, but may be suitably changed in a range
in which the ZT of the thermoelectric material may be improved.
Thereafter, the composite base material may be ground by a ball
milling method, an attrition milling method, a high energy milling
method, a zet milling method, a grinding method using a mortar, or
the like. However, a grinding method is not necessarily limited
thereto, but all methods capable of being used in the art as a
method of dry-grinding the base material to prepare powder may be
used.
Step (b) may include sintering the nanocomposite base material
powder obtained in step (a) to prepare the thermoelectric
nanocomposite.
The sintering may be performed by a sintering process generally
used in the related art, for example, a pressure-sintering process.
For example, thermoelectric nanocomposite may be prepared by a
hot-press method of putting composite base material powder into a
mold having a predetermined shape and molding the composite base
material powder at a high temperature, for example, about 500 to
900.degree. C., and a high pressure, for example, about 30 to 300
MPa. In addition, thermoelectric nanocomposite may be prepared
using a spark plasma sintering method of sintering raw materials
within a short period of time by applying a high-voltage current at
a high pressure to the composite base material powder, for example,
applying a current of about 50 to 500 A at a pressure of about 30
to 300 MPa. For example, thermoelectric nanocomposite may be
prepared by a hot-forging method of applying a high temperature,
for example, a temperature of about 500 to 900.degree. C. to
extrude and sinter thermoelectric nanocomposite base material
powder at the time of pressure-molding.
Further, the sintering may be performed at a temperature of 600 to
800.degree. C. under a pressure of 1 to 100 MPa in vacuum for 1 to
10 minutes using the spark plasma sintering method, or the like.
However, the conditions are not necessarily limited thereto, but
may be suitably changed in a range in which the figure of merit of
the thermoelectric nanocomposite may be improved.
The thermoelectric nanocomposite prepared by the sintering process
may have a density corresponding to about 70% to 100% of a
theoretical density. The theoretical density is calculated by
dividing a molecular weight by an atomic volume and may be
evaluated by a lattice constant. For example, the thermoelectric
nanocomposite may have a density of about 95 to 100%, and thus, the
thermoelectric nanocomposite may have a substantially increased
electrical conductivity.
The bulk-phase thermoelectric nanocomposite may be prepared in
various forms, and a high-efficiency thermoelectric device having a
thin thickness of 1 mm or less may be implemented. Since the
thermoelectric nanocomposite may be easily prepared as a bulk phase
and provide a high figure of merit even in the bulk phase, the
thermoelectric nanocomposite may have high commercial
applicability.
The secondary phase may be precipitated in the sintering process,
such that the thermoelectric nanocomposite may be formed.
EXAMPLE
Hereinafter, the present disclosure will be described in more
detail through the following Examples and Experimental Examples.
However, the following Examples and Experimental Examples are only
to illustrate the present disclosure, and the scope of the present
disclosure is not limited thereto.
Example 1
Preparation of
Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.03+Bi.sub.2Mg.sub.3
(Weight Ratio of Bi.sub.2Mg.sub.3 to
Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.03: 2.6%)
Precursors (e.g., metal raw materials) of a thermoelectric material
were mixed with each other at a predetermined ratio and blended in
a mortar for 10 to 30 minutes. The blended base material powder was
cold-compacted at a pressure of 10 to 90 MPa, thereby preparing a
precursor pellet. Thereafter, the compacted precursor pellet was
charged into a boat quartz and heat treated in a circular furnace
in vacuum for 4 to 10 hours, thereby preparing Mg--Si based
thermoelectric nanocomposite powder.
Thereafter, the composite base material was prepared to powder
having a size of 50 .mu.m or less by grinding in the mortar.
Further, the prepared powder was sintered at a temperature of 600
to 800.degree. C. under a pressure of 1 to 100 MPa in vacuum for 1
to 10 minutes using a spark plasma sintering method.
As illustrated in FIG. 2, in the prepared thermoelectric material,
structures of a Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.03 matrix
and a Bi.sub.2Mg.sub.3 nanoinclusion present in the matrix may be
confirmed. As a result of inductive coupled plasma (ICP) analysis,
the entire composition of the prepared thermoelectric material was
Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.03+Bi.sub.2Mg.sub.3
(weight ratio of Bi.sub.2Mg.sub.3 to
Mg.sub.1.98Al.sub.0.02Si.sub.0.97Bi.sub.0.03: 2.6%).
Example 2
Preparation of
Mg.sub.1.98Al.sub.0.02Si.sub.0.96Bi.sub.0.04+Bi.sub.2Mg.sub.3
(Weight Ratio of Bi.sub.2Mg.sub.3 to
Mg.sub.1.98Al.sub.0.02Si.sub.0.96Bi.sub.0.04: 4.0%)
A thermoelectric nanocomposite was prepared by the same method as
in Example 1.
Example 3
Preparation of
Mg.sub.1.96Al.sub.0.04Si.sub.0.97Bi.sub.0.03+Bi.sub.2Mg.sub.3
(Weight Ratio of Bi.sub.2Mg.sub.3 to
Mg.sub.1.96Al.sub.0.04Si.sub.0.97Bi.sub.0.03: 1.7%)
A thermoelectric nanocomposite was prepared by the same method as
in Example 1.
Example 4
Preparation of
Mg.sub.1.96Al.sub.0.04Si.sub.0.96Bi.sub.0.04+Bi.sub.2Mg.sub.3
(Weight Ratio of Bi.sub.2Mg.sub.3 to
Mg.sub.1.96Al.sub.0.04Si.sub.0.96Bi.sub.0.04: 2.4%)
A thermoelectric nanocomposite was prepared by the same method as
in Example 1.
Comparative Example 1
Journal of Physics and Chemistry of Solids 75 (2014) 984
Precursors (for example, metal raw materials) of a thermoelectric
material were mixed at a predetermined ratio and then ball-milled
for 1 hour. Next, a precursor pellet was prepared by
cold-compacting the mixture under a pressure of 500 Mpa. The
precursor pellet was heat treated in vacuum for 1 hour, thereby
preparing Mg--Si based thermoelectric material powder.
Thereafter, the prepared Mg--Si based thermoelectric material
powder was sintered at a temperature of 700 to 900.degree. C. under
a pressure of 1 to 100 MPa in vacuum for 30 to 100 minutes using a
hot press sintering method.
Experimental Example 1
Measurement of Electrical Conductivity, Seebeck Coefficient, and
Power Factor
Electrical conductivities and Seebeck coefficients of the
thermoelectric materials prepared in Comparative Example and
Examples 1 to 4 were measured using ZEM-3 (ULVAC-RIKO Inc.), and
the results are shown in FIGS. 3 and 4, respectively. Power factors
were calculated from the measured electrical conductivities and
Seebeck coefficients, and the results are illustrated in FIG.
5.
In Examples 1 to 4, the electrical conductivities were low, but the
Seebeck coefficients were high as compared to Comparative Example.
It may be determined that the results were caused by the carrier
filtering effect by band bending. Therefore, as shown in FIG. 5,
when the power factors were calculated, the power factors of the
thermoelectric materials prepared in Examples 1 to 4 were increased
by about 20% as compared to the thermoelectric material prepared in
Comparative Example.
Experimental Example 2
Measurement of Thermal Conductivities, Power Factors, and Figure of
Merit (ZT)
Thermal conductivities were calculated from thermal diffusivities
measured using Netzsch LFA-457 (Laser Flash method), and the
results were illustrated in FIG. 7. Lattice thermal conductivities
and thermoelectric figures of merit (ZTs) calculated from the
results are shown in FIGS. 8 and 9, respectively.
As shown in FIGS. 7 and 8, it may be confirmed that the thermal
conductivities were decreased in Examples 1 to 4 as compared to
Comparative Example. It may be determined that the thermal
conductivities were decreased by phonon scattering in a phase
boundary. Therefore, when the thermoelectric figures of merit were
finally calculated as shown in FIG. 9, at the time of comparing
thermoelectric materials prepared in Examples 1 to 4 with the
thermoelectric materials prepared in Comparative Example, the
thermoelectric figures of merit were increased by about 45% by an
increase in power factor and a decrease in thermal
conductivity.
Analysis Example 1
Deduction of Pisarenko Plot
Pisarenko plots having a correlation with a Seebeck coefficient
were deduced from carrier concentrations measured using a van der
Pauw configuration method, and shown in FIG. 6.
It may be confirmed that effective mass values of electrons were
changed by deformation of electron structures of the thermoelectric
materials prepared in Comparative Example and Examples 1 to 4
according to exemplary embodiments of the present invention.
As described above, according to the exemplary embodiments of the
present invention, the thermoelectric nanocomposite containing the
matrix composed of Mg, Si, Al, and Bi components and the
nanoinclusion composed of Bi and Mg components, such that the
nanoinclusion may be formed in the matrix as being embedded.
Accordingly, a high-density phase boundary may be formed between
the matrix phase and the secondary phase, such that the
thermoelectric nanocomposite may have a lattice thermal
conductivity decreased by phonon scattering in this phase boundary
in addition to a Seebeck coefficient increased due to the carrier
filtering effect by band bending in this phase boundary.
Accordingly, an increase in Seebeck coefficient and a decrease in
lattice thermal conductivity may be simultaneously implemented,
whereby the figure of merit of the thermoelectric nanocomposite may
be increased.
Accordingly, the thermoelectric nanocomposite according to the
present invention may be usefully used to implement a
thermoelectric device requiring high thermoelectric efficiency.
Hereinabove, although the present disclosure has been described
with reference to exemplary embodiments and the accompanying
drawings, the present disclosure is not limited thereto, but may be
variously modified and altered by those skilled in the art to which
the present invention pertains without departing from the spirit
and scope of the present invention claimed in the following
claims.
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